package Misc;

import java.awt.Color;

/**
 * @brief A Java implementation of the offcial W3 documented procedure to calculate contrast ratio
 *     between colors on the web. This is used to calculate the readability of a foreground color on
 *     top of a background color.
 * @since 2020-10-15
 * @see [Color Contrast Ratio](https://www.w3.org/TR/WCAG20-TECHS/G17.html#G17-procedure)
 * @author [Seth Falco](https://github.com/SethFalco)
 */
public class ColorContrastRatio {

  /**
   * @brief Calculates the contrast ratio between two given colors.
   * @param a Any color, used to get the red, green, and blue values.
   * @param b Another color, which will be compared against the first color.
   * @return The contrast ratio between the two colors.
   */
  public double getContrastRatio(Color a, Color b) {
    final double aColorLuminance = getRelativeLuminance(a);
    final double bColorLuminance = getRelativeLuminance(b);

    if (aColorLuminance > bColorLuminance)
      return (aColorLuminance + 0.05) / (bColorLuminance + 0.05);

    return (bColorLuminance + 0.05) / (aColorLuminance + 0.05);
  }

  /**
   * @brief Calculates the relative luminance of a given color.
   * @param color Any color, used to get the red, green, and blue values.
   * @return The relative luminance of the color.
   * @see [More info on relative
   *     luminance.](https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef)
   */
  public double getRelativeLuminance(Color color) {
    final double red = getColor(color.getRed());
    final double green = getColor(color.getGreen());
    final double blue = getColor(color.getBlue());

    return 0.2126 * red + 0.7152 * green + 0.0722 * blue;
  }

  /**
   * @brief Calculates the final value for a color to be used in the relative luminance formula as
   *     described in step 1.
   * @param color8Bit 8-bit representation of a color component value.
   * @return Value for the provided color component to be used in the relative luminance formula.
   */
  public double getColor(int color8Bit) {
    final double sRgb = getColorSRgb(color8Bit);
    return (sRgb <= 0.03928) ? sRgb / 12.92 : Math.pow((sRgb + 0.055) / 1.055, 2.4);
  }

  /**
   * @brief Calculates the Color sRGB value as denoted in step 1 of the procedure document.
   * @param color8Bit 8-bit representation of a color component value.
   * @return A percentile value of the color component.
   */
  private double getColorSRgb(double color8Bit) {
    return color8Bit / 255.0;
  }

  /**
   * You can check this example against another open-source implementation available on GitHub.
   *
   * @see [Online Contrast
   *     Ratio](https://contrast-ratio.com/#rgb%28226%2C%20229%2C%20248-on-rgb%2823%2C%20103%2C%20154%29)
   * @see [GitHub Repository for Online Contrast Ratio](https://github.com/LeaVerou/contrast-ratio)
   */
  private static void test() {
    final ColorContrastRatio algImpl = new ColorContrastRatio();

    final Color black = Color.BLACK;
    final double blackLuminance = algImpl.getRelativeLuminance(black);
    assert blackLuminance == 0 : "Test 1 Failed - Incorrect relative luminance.";

    final Color white = Color.WHITE;
    final double whiteLuminance = algImpl.getRelativeLuminance(white);
    assert whiteLuminance == 1 : "Test 2 Failed - Incorrect relative luminance.";

    final double highestColorRatio = algImpl.getContrastRatio(black, white);
    assert highestColorRatio == 21 : "Test 3 Failed - Incorrect contrast ratio.";

    final Color foreground = new Color(23, 103, 154);
    final double foregroundLuminance = algImpl.getRelativeLuminance(foreground);
    assert foregroundLuminance == 0.12215748057375966
        : "Test 4 Failed - Incorrect relative luminance.";

    final Color background = new Color(226, 229, 248);
    final double backgroundLuminance = algImpl.getRelativeLuminance(background);
    assert backgroundLuminance == 0.7898468477881603
        : "Test 5 Failed - Incorrect relative luminance.";

    final double contrastRatio = algImpl.getContrastRatio(foreground, background);
    assert contrastRatio == 4.878363954846178 : "Test 6 Failed - Incorrect contrast ratio.";
  }

  public static void main(String args[]) {
    test();
  }
}
